This invention relates to control systems for gas burning apparatus and more particularly to an improved fuel control system for the reliable and safe operation of gas burning apparatus of the type in which the supply of gas to a pilot burner is through a first valve and the supply of gas to a main burner is through the first valve and a second valve downstream of the first valve.
In some types of gas burning apparatus, it has been a common practice to employ a continuously burning pilot flame for igniting the main burner. To obviate the need for such a gas-wasting operation of a pilot burner, various automatic fuel control systems have been proposed for directly igniting the main burner of gas burning apparatus whenever operation of the apparatus is required. Such a control system is described in the Newport et al U.S. Pat. No. 3,758,260 issued Sept. 11, 1973. The Matthews U.S. Pat. No. 3,918,881 issued Nov. 11, 1975 discloses a control system for a gas burning apparatus having a pilot valve and a main valve serially connected in the gas supply of a main burner which control system establishes a pilot flame each time operation of the gas burning apparatus is required and subsequently supplies gas to the main burner to be ignited by the pilot flame. This control system includes switching means responsive to the pilot flame which are operative in the absence of a pilot flame to cause opening of only the pilot valve and energization of ignition means for the pilot burner. Upon ignition of the pilot burner, the relay switching means are operative to establish a limited-current holding circuit maintaining the pilot valve open and to simultaneously cause opening of the main valve. A warp switch responsive to full-current energization of the pilot valve is effective to shut down the control system if ignition of the pilot burner does not occur within a predetermined time interval during a start-up operation. Although this control system has certain fail-safe characteristics, any pilot flame simulating failure of the control system such as a failure of the silicon controlled rectifier in a shorted condition could cause an undesirable supply of gas to the main burner with no pilot flame to ignite the gas.